The present invention relates in general to power storage batteries. In particular, the present invention relates to a method and apparatus for estimating the charge present in a cellular telephone battery.
Cellular networks utilize ground-based transmission stations to transmit and receive modulated voice and paging signals to and from outlying mobile cellular telephone units. With the advent of more efficient and inexpensive mobile units, cellular networks have moved toward replacing conventional terrestrial networks. To consumers, cellular telephones are convenient because they may be used in remote areas without supplemental power or land-line connections. A handheld mobile unit's ability to transmit and receive signals effectively, however, depends significantly on dependable battery power within the unit. In order to increase the amount of useful battery power for cellular telephone mobile units, several battery technologies have been developed, including Nickel-Cadmium (Ni--Cd), Nickel Metal-Hydride (NiMH) and Lithium Ion (Li-Ion) chemistries. Each type of battery chemistry exhibits different charging and discharge characteristics and nominal voltages.
Useful battery power, or charge, is referred to in the present application as "ampere-hour capacity" or "capacity." This useful power is instilled into the battery during a charging cycle, wherein current is run through the battery and charge is accumulated therein. Ampere-hour capacity is the charge on a battery that can be delivered by the battery up to the limit to which the battery may be safely discharged. Battery capacity is measured in ampere-hours, which corresponds to the quantity of electricity obtained by integrating current flow in amperes over the time in hours for its flow.
For a typical cellular telephone battery, a finite amount of the battery capacity is available for use in the mobile unit "standby" mode. In this mode, the mobile unit is typically waiting for a call or a textual paging message. In this mode, the telephone may receive or transmit "superframes," which comprise sequences of modulated signal data of a specified bit length. All or a portion of this capacity may also be used for "talk" mode communications, wherein the user is linked with a base station and transmitting and receiving voice signals. This mode requires significant transmission power, and therefore almost always discharges the battery at a faster rate than the "standby" mode. For example, for a particular fully charged battery, the mobile unit may be able to operate in "standby" mode for a cumulative total of 75 hours, but may be able to operate in "talk" mode for a cumulative total of 3 hours. If the mobile unit is utilized in talk mode and standby mode at various instances in time, the total amount of available power time ("talk time" or "standby time") remaining will vary depending on the total amount of power discharged in the various modes.
It is desirable for the user to be informed as to how much talk time, standby time, or available, useful power time remains in a battery-operated communication device, such as a cellular phone, when the battery is being charged or in use. In the past, several methods have been used to indicate how much power is being deposited into or taken out of a battery. These methods, however, tended to magnify inaccuracies after a succession of charge/discharge cycles where the battery capacity measurement was not completely accurate.
Indeed, such inaccuracies often occurred because battery capacity is affected by many independent factors such as its power output rating, temperature, and age. For example, even though a cellular telephone battery is fully charged and would normally yield 90 minutes of talk time at room temperature, it may only power a mobile unit for 10 minutes if the battery temperature is -25 degrees Celsius. Moreover, a new battery may yield 90 minutes of talk time, while a battery with 200 or more charge/discharge cycles may yield only 70 minutes of talk time. Even the rate of discharge can affect the remaining capacity of the battery. For example, the remaining battery capacity available when the mobile unit is operating in "talk" mode may be 10% less than that available when the unit is operating in "standby" mode.
Therefore, there is a need for a method and apparatus for determining the capacity available in a battery in order to effectively communicate the amount of remaining usable time for the device to the user.
Furthermore, there is a need for a method and apparatus for determining this capacity based on the charge input into the battery.